Understanding Storm Track Position and Intensity Across a Range of Timescales

锋面天气系统通过之前的温暖天气和之后的寒冷天气是美国大陆和其他中纬度地区冬季天气的常见特征。这种交替不仅对当地天气很重要，而且对全球热量和水分的输送也很重要，因为温暖潮湿的空气从热带穿过中纬度地区到达两极，而寒冷和干燥的空气则从相反的方向经过。这种南北热量和水分的交换在地球气候中起着至关重要的作用，使两极不会变得太冷，同时限制了热带地区的温度过高。天气系统的这种气候影响通常是通过观察中纬度风暴轨迹来研究的，中纬度风暴轨迹被定义为天气系统活动最大的区域。在北半球，独立的风暴路径集中在北太平洋和北大西洋，而在南半球，连续的风暴路径在“咆哮的40度”周围延伸到不间断的南大洋。该项目旨在了解风暴路径的平均纬度以及沿风暴路径移动的天气系统的能量传输强度如何随着外部因素的变化而变化，这些外部因素包括太阳加热的季节周期、温室气体浓度、平流层气溶胶、冰期周期以及大陆和海洋的分布。这项研究是使用由首席研究员（PI）开发的理论框架进行的，其中风暴路径的特征是通过湿静态能（MSE）通量，其中MSE是一种能量度量，包括与气温相关的热能，凝结过程中作为热量释放的水蒸气的潜在能量，以及空气包裹在重力作用下提升的势能。风暴路径中的MSE通量与大型静止压力中心（如阿留申低压和百慕大高压）周围环流中的MSE通量（称为静止波）以及绕全球经向翻转环流（哈德利和费雷尔环流）协同工作，将能量从热带输送到两极，并平衡大气能量收支。以往关于风暴路径的研究大多是通过动量收支和相关量（例如位涡和Eliassen-Palm通量）的分析来进行的，因此，本研究的MSE分析提供了一种新颖和互补的视角。PI及其同事的初步工作表明，风暴路径中MSE通量与驻波之间的补偿是季节周期和中纬度对厄尔尼诺事件的响应的一个突出特征，而风暴路径中MSE通量与经向翻转环流之间的补偿出现在对温室气体和气溶胶辐射效应的大气响应的模拟中。目前的工作扩展了这些调查，并试图确定这些补偿的基本动力学，其中风暴路径MSE通量的变化伴随着其他形式的MSE通量的相反变化。这项工作是通过将观测分析和模拟与不同复杂程度的数值模式结合起来进行的。由于风暴路径行为对包括美国大陆在内的中纬度地区的人类活动的重要性，这项工作具有社会意义和科学意义。此外，该研究的一个目标是确定可用于评估气候变化模拟可信度的“紧急约束”。紧急约束是在模拟当前气候（预测者）的模式间变化和模拟未来气候变化（预测者）的模式间差异之间出现的关系。这些限制被用来评估气候变化预估的可信度，因为模型如果不正确地代表预测者，可以与现实世界的观测结果进行比较，那么在模拟未来气候变化时可能会产生类似的错误陈述。鉴于利用气候模式为适应气候变化的努力提供信息，这种对气候变化预估的测试是有价值的。该项目还支持一名研究生和一名本科生的参与，从而为该研究领域的未来劳动力提供教育和培训。

Warm days preceding and cold days following the passage of frontal weather systems are a familiar feature of winter weather in the continental US and other middle latitude regions. This alternation is important not only for local weather but for the global transport of heat and moisture, as warm moist air crosses the middle latitudes from the tropics to the poles while colder and drier transits in the opposite direction. This north-south exchange of heat and moisture plays an essential role in earth's climate, keeping the poles from getting too cold while limiting the temperature excess of the tropics. Such climatic effects of weather systems are commonly studied by looking at the midlatitude stormtracks, defined as zones of maximum weather system activity. In the Northern Hemisphere separate stormtracks are centered over the northern North Pacific and North Atlantic, while in the Southern Hemisphere a continuous stormtrack extends around the "roaring 40s" over the uninterrupted Southern Ocean.This project seeks to understand how the mean latitude of the stormtracks, and the and strength of the energy transport by the weather systems moving along them, change in response to external factors including the seasonal cycle of solar heating, greenhouse gas concentrations, stratospheric aerosols, ice age cycles, and the distribution of continents and oceans. The research is conducted using a theoretical framework developed by the Principal Investigator (PI) in which stormtracks are characterized in terms of the flux of moist static energy (MSE) across them, where MSE is a measure of energy which includes the thermal energy associated with air temperature, the latent energy of water vapor released as heat during condensation, and the potential energy of air parcels lifted against the force of gravity. The MSE flux in stormtracks works in concert with MSE flux in circulations around large stationary pressure centers such as the Aleutian Low and the Bermuda High (referred to as stationary waves), and the circumglobal meridional overturning circulations (the Hadley and Ferrel cells), to transport energy from the tropics to the poles and balance the atmospheric energy budget. Much of the previous research on stormtracks has been conducted through analysis of the momentum budget and related quantities (potential vorticity and Eliassen-Palm flux, for instance), thus the MSE analysis of the present work offers a novel and complementary perspective.Preliminary work by the PI and colleagues shows compensation between MSE flux in stormtracks and stationary waves as a prominent feature of the seasonal cycle and the midlatitude response to El Nino events, while compensation between MSE flux in stormtracks and meridional overturning circulations appears in simulations of the atmopsheric response to greenhouse gases and the radiative effects of aerosols. The present work extends these investigations and seeks to determine the fundamental dynamics through which these compensations, in which changes in stormtrack MSE flux are accompanied by opposing changes in other forms of MSE flux, come about. The work is conducted through a combination of observational analysis and simulations with numerical models at varying levels of complexity.The work has societal relevance as well as scientific interest due to the importance of stormtrack behavior for human activities in the midlatitudes, including the continental US. In addition, one goal of the research is the identification of "emergent constraints" which can be used to assess the credibility of climate change simulations. Emergent constraints are relationships that emerge between model-to-model variations in simulated present-day climate (the predictor) and model-to-model differences in simulated future climate change (the predictand). Such constraints are used to assess the credibility of climate change projections, as models which incorrectly represent the predictor, which can be compared to real-world observations, are likely to produce comparable misrepresentations in their simulations of future climate change. Such tests of climate change projections are valuable given the use of climate models to inform climate change adaptation efforts. The project also supports a graduate student and the participation of an undergraduate, thereby providing for the education and training of the future workforce in this research area.